The present invention relates to the control of electric motors, and in particular to measurement of current in electric motors and the control of electric motors based on current measurement.
A closed loop current controller for an electric actuator typically takes the form shown in FIG. 1, where, using vector control, a voltage is applied to each phase winding of the motor and the resultant generated currents measured. The measured currents are then used by a controller, typically a PI, to control to the requested target current. A signal which gives the electrical position of the motor is also required so that the voltages can be applied at the correct phases at the correct times. In the system of FIG. 1 a position sensor is shown but the present invention is equally applicable to a position sensorless system, where the position signal is estimated from other sensor measurements.
Due to inaccuracies and limitations of the current sensor the measurement process can introduce harmonics into the output signal of the current sensor that are not present in the currents that are being measured. The current controller then reacts to these harmonics in the sensor output to try to remove them, but their removal by the current controller, to give a smooth output from the current sensor, causes the actual currents to contain these harmonics.
Depending on the frequency and amplitude of these unwanted harmonics the torque produced by the motor can be degraded, with the harmonics potentially causing torque ripple and/or acoustic noise.
Current Measurement
Typically two methods of phase current measurement are employed:                1. Phase current sensors, where a current measurement device is placed in each of the phases. (For a three phase system it may be that only 2 phases are measured as the 3rd phase can be calculated from the 2 measured phases)        2. Single current sensor, where the current flowing in the DC link is measured at specific points during the PWM duty cycle to allow the current in the 3 phases to be calculated.        
The harmonics introduced will be different, depending on the phase current measurement method used. The present invention is applicable to any harmonic introduced by measurement inaccuracies.
Effect of Current Measurement Error
The effect of a distortion on the feedback measurement signal in any closed loop system is well known from standard control theory. The effect is described explicitly below for the case of a motor current controller.
The distortion on the current measurement appears as an unknown disturbance on the current feedback path as shown in FIG. 2. The disturbance will add harmonics to the ‘measured current’ signal from the sensor. If these disturbance harmonics fall within the bandwidth of the current controller, the current controller will act to reject them by modulating the control (voltage demand) signal in order to cancel them out. This modulation will cause the harmonics to appear on the actual phase currents, even though the harmonics are no longer present in the measured phase current signals. This will lead to unwanted torque ripple at the motor output shaft.